The invention relates generally to fiber optic sensors, and more particularly to fiber optic sensors for measuring refractive index.
A fiber optic is an optical waveguide which transmits light by total internal reflection (TIR) at the core/clad interface. The critical angle A.sub.c for TIR is determined by the ratio of the refractive index N.sub.2 of the clad to the refractive index N.sub.1 of the core: A.sub.c =sin.sup.-1 (N.sub.2 /N.sub.1). Thus the index of the clad must be less than the core for TIR to occur.
Optical fibers have been used in a wide variety of sensors, known as "optrodes" or "fiber optic chemical sensors" (FOCS). which are designed to measure the presence of various chemical species or the value of various parameters such as pressure or temperature. In most cases a signal from a reactant, e.g. a fluorescent signal from a fluorescent dye which interacts with the desired chemical species or is affected by the desired physical parameter, is transmitted through the fiber to a detector. These sensors are generally limited by being specific to a single chemical species or physical parameter; thus each sensor is based on its own unique chemistry. U.S. Pat. No. 4,846,548 issued July 11, 1989 is directed to a more generalized fiber optic sensor methodology in which the principle of detection is based on how the operating characteristics of the fiber itself are modified as the result of the presence of the desired species. The use of this sensor principle allows the fabrication of many different sensors which are sensitive to particular species or even groups of species. However, it would also be desirable to have available a more general sensor which can detect and differentiate a wide variety of species. Since different species usually have different refractive indexes, a single sensor which can measure refractive index would be able to detect the presence of different species. Thus, such a sensor would not be species specific but would be a more universal detector.